Preliminary Analysis of PEN Experiment: A Precision Measurement of the Pion Electronic Decay Branching Ratio

Lepton universality is a key hypothesis of the Standard Model of particle physics which predicts that the charged leptons couple equally in electroweak interactions. All electroweak interactions with the charged leptons are therefore identical under the umbrella of lepton universality. This hypothesis may be confirmed through either high energy collisions, or low energy precision experiments. The PEN experiment, performed at the Paul Scherrer Institute, sought to obtain the pion electronic decay branching ratio $\Gamma(\pi\rightarrow \text{e}\nu (\gamma)/\Gamma(\pi \rightarrow \mu\nu(\gamma))$ to a relative precision of $5\times 10^{-4}$. This ratio provides the best test for electron-muon universality. Deviations from Standard Model expectation would be indicative of physics beyond the Standard Model, or ``new physics''. Confirmation of the Standard Model expectations put limits on new physics. This dissertation describes a blinded cut-based preliminary blinded analysis for the period comprising the 2009 and 2010 data taken by the PEN collaboration. This dissertation includes the reconstruction of key observables from detector response, the construction and implementation of an ultra-realistic Monte Carlo simulation, and the necessary event selection criteria in order to obtain the branching ratio.
Emphasis is placed on the construction of the Monte Carlo simulation needed for the characterization of the low energy tail of the CsI electromagnetic calorimeter as well as determination of the acceptances and cut efficiencies used in the analysis. Characterization of various quantities and uncertainties needed to obtain the branching ratio is also discussed. The partial unblinding of the analysis when including key cut efficiencies is $(1.2534 \pm 0.0013)\times 10^{-4}$ with a relative statistical uncertainty of $8.3\times 10^{-4}$ for the 2009 run period. Full unblinding is expected to lower the branching ratio value on the order of 2\% . The blinded branching ratio for the 2010 run period results in a $(1.1751\pm0.0010)\times 10^{-4}$ result with a relative statistical uncertainty of $6.7\times 10^{-4}$. Combining the two years of the data results in $5.3\times 10^{-4}$ relative statistical uncertainty and an overall relative uncertainty of $7.6\times 10^{-4}$.
With further study on acceptances, event rejection criteria, and inclusion of the 2008 data, the statistical uncertainty will be diminished to produce a more competitive result with the inclusion of higher statistics.